1. Field of the Invention
The present invention relates to communication apparatuses, communication methods, and communication systems using millimeter waves to perform wireless communication and, more particularly, to a communication apparatus, a communication method, and a communication system capable of securing appropriate coordination with multiple communication partners performing millimeter-wave communication.
2. Description of the Related Art
Very high-speed data communication can be realized by wireless communication using millimeter waves, which uses a wavelength of 10 mm to 1 mm and a frequency of 30 GHz to 300 GHz and which can allocate channels in units of GHz. For example, in Institute of Electrical and Electronics Engineers (IEEE) 802.11ad, standardization of wireless local area network (LAN) standard using a 60 GHz band is being advanced. Since the millimeter waves have shorter wavelengths and higher linearity, compared with microwaves being in widespread use in, for example, wireless LAN technologies, a very large amount of information can be transmitted by using the millimeter waves. However, since reflection of the millimeter waves is accompanied by strong attenuation, direct waves and waves reflected no more than once are mainly transmitted on the wireless communication path. In addition, since the millimeter waves have higher transmission loss, the millimeter waves have the property that radio signals are not transmitted to a distance.
In order to resolve such a communication distance problem of the millimeter waves, methods are considered in which directivity is given to the antennas of transmitters-receivers and the transmission beams and the reception beams of the transmitters-receivers are directed to directions where communication partners are positioned to increase the communication distance. However, the methods have drawbacks in that signals do not reach neighboring stations at directions other than the directions of the communication partners although the transmission-reception power is increased at the directions of the communication partners. Accordingly, the directional communication is not appropriate for coordination between multiple communication stations.
In IEEE 802.11n, directional communication using microwaves having a frequency band of 2.4/5 GHz is defined. In this directional communication, packets for the coordination are transmitted at a rate lower than that of data packets to allow scheduling information to be transmitted to neighboring stations within a wider range. However, a sufficient amount of scheduling information may not reach the neighboring stations at a frequency band of 60 GHz even if the packets are transmitted at a lower rate.
In addition, wireless communication apparatuses are proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-188925), in which search tones are transmitted by using the millimeter waves while the radiation directions of directional antennas are reciprocated and rotated at a constant angular velocity in 360 degrees on the horizontal plane and communication nodes are searched for on the basis of response tones returned from the communication nodes that have received the search tones. However, it takes a time to reciprocate and rotate the search tones to search for the communication nodes in 360 degrees.